1. Field of the Invention
The invention relates to eddy current non-destructive examination (NDE) and evaluation of physical properties of a component, such as a generator retaining ring, rotor forging, rotor wedge, damper bar or other generator component, after experiencing potentially degrading thermal exposure during any stage of manufacture, assembly and service use. In this invention eddy current test measurements are correlated with component thermal exposure (e.g., absolute temperature and/or cumulative time-temperature heat absorption) and cumulative alteration of the component physical properties, such as, among others, material yield strength (YS), toughness, and tensile ductility. In this invention, using the eddy current test measurements and reference data correlating electrical conductivity with ring material thermal exposure, the component's physical properties are evaluated to determine whether it is acceptable for service use, requires further modification (e.g., additional heat treatment processing) or whether is permanently unsuitable for service. The invention test method can be employed with in situ components, such as generator rings within field serviced generators, without altering the component structure.
2. Description of the Prior Art
FIGS. 1 and 2 show a known generator 20, having a generator rotor 22 with rotor windings, including end turn bends 24. A slip layer liner 26 covers the end turn bends 24 and one or more locking keys 28 project radially from the rotor circumference. Electrical generator retaining rings 30 at each axial end of the rotor 22 support the rotor winding end turns 24 against centrifugal force generated by rotor cyclic rotation, and thus are subjected to annularly oriented stress. Thus the retaining rings 30 are designed to be tolerant to high stress levels while possessing adequate low cycle fatigue and toughness. The retaining rings 30 are generally constructed of non-magnetic material, in order to minimize rotor end leakage flux, which also reduces rotor operating temperature. Exemplary known materials used to construct the generator retaining rings 30 include 8Mn-8Cr-4Ni austenitic steel, 9Mn-6Cr-4Ni austenitic steel and 18Mn-5Cr austenitic steel. In recent decades 18Mn-18Cr austenitic steel has become commonly used due to its higher corrosion resistance and lessened likelihood of stress corrosion cracking while the generator 20 is in service.
Generally, the retaining rings 30 are shrink-fitted (by application of heat to expand the rings and subsequent shrinkage after cooling) onto the end of the generator rotor 22 body, with an interior annular surface formed ring groove 32 interlocking with generator body locking key(s) 28. This axial interlocking, prevents ring 30 separation from the generator rotor body 22 as the rotor windings and their end turns 24 expand in the axial direction as they are internally heated by the electromagnetic forces generated within the generator 20. The resultant shrink fit also creates an additional mechanical stress within the retaining rings 30, which is anticipated and compensated for in the ring design parameters. However, thermal degradation of the retaining ring 30 structure physical properties during shrink fit heating and service operational heating is more difficult to anticipate and compensate for in the ring design. It is even more challenging to determine the extent of thermal degradation impact on previously in-service retaining rings 30 and other generator structures physical properties when a generator 20 is periodically inspected serviced after many hours of field operation.
Retaining ring 30 material physical properties, such as such as, among others, material yield strength (YS), toughness, and tensile ductility change are altered by thermal exposure; whether absolute temperature exposure or time-temperature exposure. Generator retaining ring 30 adverse thermal exposures due to accidental overheating in shrink fit assembly or service can result in significant loss of its material physical properties. There exist critical absolute temperature and cumulative time exposure limits above which such thermal exposure can adversely impact generator ring 30 performance. However, there is no reliable and quantitative non-destructive method to evaluate the level of overheating and determine if such deleterious thermal exposure has occurred.
In the past, engineers have relied on “temper colors” or surface tints to estimate temperature of exposure. This is not very reliable, as it is quite subjective and also depends on other variables such as initial surface condition, exposure time and environment. Microstructural evaluation by in-situ metallography is presently used but is time-consuming and often generates ambiguous test results. In the absence of reliable and repeatable evaluation of generator retaining ring 30 thermal exposure, manufacturers and users tend to scrap retaining rings 30 or other generator structures that are suspected of any degree of overheating, rather than risk future potential deleterious field service. Scrapping generator components and replacing with new rings entails significant cost and time delays.
Known NDE of an industrial object by an eddy current modality identifies discontinuities, such as cracks or voids, by passage of a steady state alternating current or pulsed current waveform in a test probe transmitter coil that is electromagnetically coupled in close proximity to an electrically conductive test object. The changing current flow in the probe transmitter generates a changing transmitted magnetic field waveform that in turn induces a generated eddy current in the electromagnetically coupled test object. Variations in the phase and magnitude of these generated eddy currents within the test object create a responsive or reflected magnetic field waveform that is in turn sensed by a test probe receiver coil as an induced received or reflected current flow. In some known eddy current NDE systems the test probe's transmitter coil also functions as the receiver coil. Thus, variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the reflected magnetic waveform as sensed by the test probe receiver changes in its current waveform. Amplitude and intensity of an eddy current within a test object will stay substantially constant if its magnetic transmission characteristics (which impact the reflected waveform) are substantially constant. Anomalies in the test object alter its magnetic transmission characteristics at the anomaly location. Accordingly, anomalies and their spatial location within the test object can be detected by determining if the magnetic transmission characteristics of the material being scanned are consistent with the presence or absence of an anomaly at each scan spatial location. However, a thermally degraded test component may not have any internal flaws that can be detected by known eddy current NDE methods.
Thus, a need exists for a non-destructive examination (NDE) and evaluation of the physical properties of components, such as generator retaining rings or other generator components, after they experience potentially degrading thermal exposure during any stage of manufacture, assembly and service use.
A separate additional need exists for in situ non-destructive examination (NDE) and evaluation of the physical properties of components, such as generator retaining rings or other generator components, after they experience potentially degrading thermal exposure, so that the component does not need to be separated from its operative environment.